What in heaven's name is an abstract base class?

21 Dec, 2014

Let's talk about duck-typing for a second. Duck-typing is an object-oriented programming principle that allows an object's methods and properties to handle the semantics and behavior of an object without the need to determine what type of object it actually is. The idea is that "if it looks like a duck and quacks like a duck, it's a duck" and the programmer doesn't need to first ask the object if it's actually a duck. We can just start asking it to quack.

An example:

[1, 2, 3, 4, 5].len() # 5
"hello".len()		  # 5

I don't need to ask the object if it's a string or a list before calling len(). I'm trusting that the object's len() method already knows what to do.

Abstract base classes complement duck-typing by providing an alternative way to define an interface when all of the possible use cases for an object might not have been anticipated by the designer. This scenario might pop up in a large development team, or if a software plugin might be written by an external team. In theory, we create an abstract base class by defining a class and marking its methods as abstract, then registering a concrete class with the interface or by simply subclassing the abstract class.

Confused yet? Don't worry, my head's spinning a bit too. Let's continue.

I define an abstract base class like so:

import abc

class Animal:
	__metaclass__ = abc.ABCMeta

	@abc.abstractmethod
	def greet(self):
		return

With the __metaclass__ = abc.ABCMeta declaration, we've defined Animal as an abstract base class. We've also defined an abstract method, greet(), that doesn't do anything but return. That's totally okay - anything that inherits from Animal will need to write their own greet() anyway, as concrete classes can't access those abstract methods directly. Let's finish this out:

import abc

class Animal:
	__metaclass__ = abc.ABCMeta

	@abc.abstractmethod
	def greet(self):
		return

class Lion:

	def greet(self):
		return "ROAR"

Animal.register(Lion)

print('Subclass:', issubclass(Lion, Animal))
print('Instance:', isinstance(Lion(), Animal))

Our Lion implements the Animal API, but it's not derived from Animal.

We'll see this approach again when we look at the collections module, which uses abstract base classes to test whether a collection is hashable or a mapping.